Technicians generally measure the loss of optical fiber splices on installation to ensure that the splice loss is low enough for proper system performance.
U.S. Pat. 4,652,123 discloses a known method for measuring optical fiber splice loss in the field. According to this method, first and second bent fiber couplers are attached to the spliced fibers at locations which are upstream of the splice and downstream of the splice respectively. A third bent fiber coupler is then attached to the downstream fiber between the second coupler and the splice. Optical signals propagating toward the splice are launched at the first and second couplers and detected at the third coupler as optical power measurements P.sub.21 and P.sub.12 respectively. The third coupler is then moved to a location between the first coupler and the splice, and the signals launched at the first and second couplers are detected by the third coupler as optical power measurements P.sub.11 and P.sub.12 respectively. The absolute splice loss L is then computed from the relationship: L.sup.2 =(P.sub.21 P.sub.12)/(P.sub.11 P.sub.22), where L is the splice loss stated as a linear fraction of the signal passed by the splice. The splice loss in dB is given by 10 Log L. (U.S. Pat. No. 4,652,123 was issued in the name of Richard Neumann on Mar. 24, 1987 and is entitled "Method and Apparatus for Measuring Absolute Fiber Junction Loss".)
The method disclosed in U.S. Pat. No. 4,652,123 infers the splice loss from measurements of launched and transmitted power. This indirect determination of the splice loss can be very accurate if the actual splice loss is approximately 0.5 dB or greater. However, if the actual splice loss is significantly less than 0.5 dB, it is comparable to the normal measurement error for the measurement of P.sub.11, P.sub.12, P.sub.21 and P.sub.22. Consequently, for low loss splices the normal measurement error may obscure the splice loss, and the method of U.S. Pat. No. 4,652,123 may provide an inaccurate determination of the actual splice loss.